Particle counters and sensors are used to detect light scattered by particles entrained in a stream of fluid, e.g., in an air stream. Such counters and sensors draw air (with entrained particles) from a room, for example, and flow such air along a tube and through an illuminated sensor "view volume" so as to obtain information about the number and size of such particles. Such information results from an analysis of the very small amounts of light "scattered" by the particle as it moves through the view volume.
Some types of sensors flow such air along an enclosed transparent tube; others "project" the air and accompanying particles at a particular flow rate (often measured in cubic feet per minute) from one tube across an open space to another tube. In sensors of the latter type, there is no tube wall (however transparent such wall may be) to impair light scattering and collecting. In other words, the particle is briefly illuminated by a very-small-diameter light beam as it "flies" through an open space.
Among other uses, particle counters incorporating particle sensors are used to obtain a measure of air quality by providing information as to the number and size of particles present in some specified volume of air, e.g., a cubic meter of air. Even work environments which appear to human observation to be clean--business offices, manufacturing facilities and the like--are likely to have substantial numbers of microscopic airborne particles. While such particles are not usually troublesome to the human occupants, they can create substantial problems in certain types of manufacturing operations.
For example, semiconductors and integrated chips are made in what are known as "clean rooms," the air in which is very well filtered. In fact, clean rooms are usually very slightly pressurized using extremely clean air so that particle-bearing air from the surrounding environs does not seep in. And the trend in the semiconductor and integrated chip manufacturing industry is toward progressively smaller products. A small foreign particle which migrates into such a product during manufacture can cause premature failure or outright product rejection even before it is shipped to a customer. This continuing "miniaturization" requires corresponding improvements in clean-room environments (and in the related measuring instruments) to help assure that the number and size of airborne particles are reduced below previously-acceptable levels. Known particle counters and sensors have not been entirely successful in this regard.
U.S. Pat. No. 4,606,636 (Monin et al.) patent describes a particle analyzing apparatus into which a particle is introduced through a tube-enclosed "view volume." A paraboloid reflector is shown, an ellipsoid reflector is described and the mirror cavity is obstructed by a mask. Particle-carrying tubes can refract light unpredictably and, often, the apparatus optical system is required to be more complex as a result. U.S. Pat. No. 4,523,841 (Brunsting et al.) shows a system used to measure aspects of biological cells. The system uses an ellipsoid reflector, a wide-area detector and a "cornucopia" type of light trap. U.S. Pat. No. 4,189,236 (Hogg et al.) patent shows a radiation collector used for analyzing aspects of blood cells and the like. Such collector uses light scattered by, e.g., a cell, and twice reflected before being received at a detector. U.S. Pat. No. 3,248,551 (Frommer) shows an optical arrangement for sensing very small particles such as dust or pollen.
Another disadvantage of known particle counters and sensors becomes manifest when trying to detect very small particles, e.g., 0.1 micron and smaller, and/or when trying to detect particles present in a relatively high-flow-volume, e.g., one cubic foot per minute, air stream. Because the particle is very small and/or because it is moving relatively rapidly (thus passing quickly through the illuminated view volume), such particle reflects and scatters very little light. The quantum of such light which can be detected and accurately measured is often near or below the resolution and sensitivity limits of existing sensors and detectors. And, of course, the latter produces a low level of inherent "noise."
An apparent solution is to increase the quantum of light reflected and scattered by a very small and/or fast-moving particles by increasing the intensity of the light beam. Such efforts have proven largely counterproductive since a more intense light source produces higher levels of random electronic or "shot" noise. And as the light beam becomes more intense, the quantum of light scattered by gas molecules tends to increase, irrespective of whether a particle is also present in the view volume. Shot noise and the increasing quanta of light scattered by gas molecules tends to partially or totally obscure the effect of the particle-scattered light.
To complicate matters even further, laser light sources tend to vary, even if only slightly, in output power during operation. As a consequence, the quantum of light scattered by gas molecules varies with variations in power. For high powered lasers used to detect very small particles at high flow rates, such variations can be of magnitudes much higher than the shot noise or the noise inherent in an electronic detector. These phenomena dramatically limit the attainable sensitivity.
An improved particle sensor which collects and analyzes a high percentage of light scattered by a very small particle and which helps neutralize the effect of light source power variations and gas molecule light scattering would be an important advance in the art.